Both skis are good.
You mention that you are after stability. In the stealth range the Fisha 500 would be a good option but in my opinion it will be the same as the pinnacle. Only difference is that you will have the benefit of having a lighter ski, compared to the Pinnacle. I've recently changed from a pinni to a stealth profisha 525. It is a pretty stable ski once you paddle it a few times and you have the added benefit of speed, which trust me, you will appreciate after paddling a pini. Stealth have launched the 555 which I've only heard good things about. From what I've observed, guys that have owned the Fisha 500 have upgraded to the 555 for it's stability. It's still slower then the 525, even though it's longer.
On the Vortex, these are lovely boats. They launched about a year ago and have made a couple of refinements since then. When I lived in Durban, a few guys switched from their stealth 575 to the vortex. After a while, some switched back to the 575. I guess it depends on what you preference is and what you are used to. Stability and speed were the two words that always came up when talking about the vortex. It also has a pretty large hatch space from what I've seen, which is one up on the stealth.
The deciding factor for me (I was considering a Vortex at one point) would be the after sales support and spares between the two company. Vortex are still new to the market and I reckon that they need another year or two to firmly establish themselves in the market. They are doing a really good job so far.
Price wise, they are about the same but I reckon that the stealth will hold more value if you decide to upgrade at a later point. You can get a few good second hand ones for a good price. I think Vortex has a couple of demos still for sale but from what I've heard, these are slightly heavier then the more newer version (due to all the refinements).
See if you can arrange to paddle all the skis before making a decision.
Good luck.